Aldosterone

()Map IMPORTANT HORMONAL REGULATORS OF THE KIDNEY (Map)

Overview

Aldosterone is a steroid hormone which is secreted from the zona glomerulosa of the adrenal gland. It has a mineralocorticoid activitiy and is the most important regulator of plasma potasium. When plasma potasium increases increased stimulation of aldosterone occurs directly and as a result of Renin-Angiotensin-Aldosterone System (RAAS). It is also the most important regulator of sodium excretion.

Release

Release is stimulated by 3 things

Corticotropin (ACTH)
Angiotensin 2
K⁺

Its release is inhibited by Atrial Natriuretic Peptide

Most increases in the concentration of aldosterone however can be explained by increases in the Renin-Angiotensin-Aldosterone System and therefore angiotensin 2 and/or by increases in K⁺ concentration
Only in severe fluid loss does ACTH significantly stimulate the release of aldosterone
ANP is secreted in response to sodium/water loading and therefore inhibits aldosterone secretion

Action

Diffuses across the cell membrane - lipophillic (essentially steroidal)
Of the principal cells of distal tubule and Collecting Duct
Binds to cytoplasmic receptors
Works by altering gene transcription and increases synthesis of proteins
- Affects ATP levels

Sodium

Affects sodium entry and transport
Increases number of apical sodium channels, NaCl co-transporters and Na⁺K⁺ATPase
Increases activity of the hydrogen sodium exchanger in the apical membrane
Increases membrane permeability
Increases sodium pump activity
Total quantity of sodium is conserved not the actual plasma concentration
- This is because water follows sodium so the volume is altered according to the amount of sodium
- Angiotensin 2 and aldosterone also affect ECF so only quantity affected not concentration
- ADH and thirst response also work together to dilute the ECF so although there is more NaCl the actual concentration is constant.
If there was no secretion of aldosterone a 20kg dog would excrete 15g per 24 hours
At maximal secretion no significant amount of sodium would be excreted

Potassium

In cases of increased K⁺
Increased Na⁺ / K⁺ ATPase pump activity increases the amount of K⁺ in cells to reduce plasa K⁺
Generally not excreted
However if plasma K⁺ is still high aldosterone is stimulated
Causes pottassium secretion
- Stimulates Na⁺ / K⁺ ATPases in the basolateral membrane of the principal cells
- Increased pottasium in the cells
- Pottassium leaves via apical leak channels
- Thanks to electro-chemical gradient
Very tightly regulated system
- Allows large increase in K⁺ to have a miniscule effect on plasma K⁺

Hydrogen

Increases hydrogen secretion by increasing Hydrogen ATPases in the apical membrane of the intercalated cells
Increases hydrogen secretion by increasing sodium hydrogen exchanger in the apical membrane of the principal cells